Tiny Capsules for Electronic Paper Displays

MIT's 1999 patent on a special ink made of tiny capsules that can change color when an electric field is applied, forming the basis for early e-readers.

What it covers

This patent describes a material for electronic displays, like early e-readers. It's an 'electrophoretic material' made of a carrier liquid holding many tiny capsules, called microcapsules. Inside each microcapsule are particles that can move when an electric field is applied. Some particles are designed to reflect light better, making them appear bright. Others might be colored or even emit light. The key is that these particles can be moved to show different colors or patterns, and importantly, they stay that way even after the electric field is turned off. This 'bistability' is what makes it work like paper, holding an image without constant power. For example, imagine a microcapsule with black particles and white reflective particles. Applying a field can push the white particles to the front, making the capsule look white, or push the black particles forward, making it look black.

What it doesn't cover

• —Materials that require continuous power to maintain their displayed image.
• —Displays where the particles do not have differential responsiveness to an electric field.
• —Microcapsules that do not contain multiple phases or particles with contrasting visual properties.
• —Materials where the visual appearance does not persist after the electric field is removed.
• —Displays that rely solely on light emission without reflective elements for their appearance.

The clever bit

The innovation lies in encapsulating the moving charged particles within tiny spheres and controlling their movement with an electric field, while also ensuring the image remains visible even when the field is off. This created a stable, low-power display that mimics the appearance of ink on paper.

Why it matters

This patent is foundational for the development of electronic paper (e-paper) technology. It describes the core microencapsulated electrophoretic display (EPD) technology that enabled the first generation of e-readers and other bistable displays. The ability to hold an image without power consumption was a significant leap for portable electronic devices.

Real-world examples

• 1.Early Amazon Kindle devices
• 2.E-readers from other manufacturers
• 3.Electronic shelf labels
• 4.Some smart cards
• 5.Digital signage requiring low power